Low modulus substrate for flexible flat panel display

ABSTRACT

The present invention concerns substrates for a flexible flat panel display able The present invention concerns substrates for a flexible flat panel display ( 110 ) able to bent into a low radius of curvature, wherein the cell gap deviation is controlled and preferably minimised, even when the flexible flat panel display ( 110 ) is affected by pressure application or bending. According to a certain aspect there is provided composite substrates consisting of a substrate with a high Young&#39;s modulus and a substrate of low Young&#39;s modulus.

FIELD OF INVENTION

The present invention relates to a flexible flat panel display such as aliquid crystal display (LCD), an organic light emitting diode display, afield emitting display, or a thin or thick film electro-chrome orelectro-luminescence display. In particular, the invention relates to aflexible flat panel display able to bend into a radius of curvature. Thepresent invention further relates to a flexible substrate for a flatpanel display.

BACKGROUND OF INVENTION

Flexible flat panel displays are at present in their development stages,however, an expanding market is envisaged in a wide variety ofcircumstances, where the flexible flat panel displays, in particular,experiences tensile, compressive and shear stresses while thefunctionality of the flat panel display is maintained. Duringmanufacturing of panel displays, the flat panel displays are exposed topressure loads for example during bonding of layers together, however,the flexibility of the flat panel displays ensures that the largestpossible number of flat panel displays will work.

A flexible flat panel display has been described in earlier patentapplications. For example British patent application no. GB 2 337 131 Adescribes a LCD and a manufacturing method for such, in which the LCDcomprises two layers of substrates separated by wall-shaped spacers. TheLCD is specifically designed so as to satisfy the conditionqL⁴/Eh³≦π⁵V/48, where ‘q’ is applied pressure such as bonding pressureduring manufacturing, ‘L’ is the distance between the wall-shapedspacers, ‘E’ is the modulus of elasticity of the substrate, ‘h’ is thethickness of the substrate, ‘V’ is the tolerable change in the thicknessof the cell defined between the two layers of substrates and thewall-shaped spacers. The object for satisfying the above condition theLCD is according to the British patent application to manufacture a LCDelement, which is capable of maintaining an even cell thickness (gap)during pressure applied normal to the substrate surface and providing afavourable display. Nevertheless, during a manufacturing process orduring use of the LCD, the LCD may, in addition to be exposed topressure, further be exposed to bending.

Furthermore, International patent application no. WO 02/43032 describesa flexible display device including a flexible substrate and a pluralityof row and column electrodes attached to the substrate with a displaymaterial between the row electrodes and the column electrodes. Thematerial for the substrate may be an inorganic glass or a polymer film.However, the flexible display device described in the Internationalapplication utilises amorphous and semi-crystalline polymers, which arein their glass state at normal usage conditions of the displays.

In addition, the prior art does not satisfactorily provide a substratesuitable for a flexible flat panel display allowing a low radius ofcurvature, while maintaining a satisfactory display operation.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the problems encounteredwith the prior art as described above, problems relating to allowingflexibility of flat panel display element exposed to pressure and/orbending.

The above objects, together with numerous other objects, advantages andfeatures, which will become evident from below detailed description, isobtained according to a first aspect of the present invention by aflexible flat panel display comprising a first substrate characterizedin that the first substrate has a modulus of elasticity smaller than orequal to 1.5 GPa.

The first substrate according to the first aspect of the present mayfurther have a modulus of elasticity smaller than or equal to a modulusselected among the group consisting of 1.3 GPa, 1.1 GPa, 1 GPa, 0.9 GPa,0.8 GPa, 0.7 GPa, 0.6 GPa, 0.5 GPa, 0.4 GPa, 0.3 GPa, 0.2 GPa and 0.1GPa. By utilising materials having a low modulus of elasticity aparticular flexible flat panel display may be achieved.

The first substrate may be fabricated from any polymer film in itsrubber state during normal working conditions for a typical flat paneldisplay. That is, a material having a glass transition temperature belowthe normal working conditions for a typical flat panel display, e.g.below 80° C., below 60° C., below 40° C., below 30° C., below 0° C.,below −20° C. below −40° C.

Further, the first substrate may be fabricated from any rubbers orrubber-like polymers for example based on silicone, urethane, neoprene,butyl rubber, ethene-propene rubber, acrylate rubber, butadiene rubber,choloprene rubber, nitrile rubber, 1-1 propene rubber, flouridisedrubber, styrene-butadiene, natural rubber or any combination thereof.

The flat panel display may comprise an electro-optical medium such asliquid crystal, or an electro-chrome or electro-phoretic element, alight emitting element, an organic or inorganic light emitting element,polymer light emitting element, or any combination thereof. Any type ofelectro-optical medium may be used in the flat panel display without thematerial properties are compromised thus ensuring the flexibility in aflat panel display using any type of electro-optical medium. In fact,even plasma may be used as the electro-optical medium.

The flexible flat panel display according to the first aspect of thepresent invention may further comprise one or more layers positionedsubstantially coplanar and adjacent to upper and/or lower surface of thefirst substrate. The layers may have a high modulus of elasticity, thatis, having a modulus of elasticity greater than 1.5 GPa. Alternatively,the layers may have a low modulus of elasticity, that is, a modulus ofelasticity lower equal to or smaller than 1.5 GPa. The layers mayprovide a coating to the first substrate and have a thickness up to 80%of the total thickness of the first substrate and the additional layersadded to the first substrate.

Further, the flat panel display may comprise a display substratepositioned coplanar with the first substrate, which display substratemay have a modulus of elasticity smaller than or equal to 1.5 GPa. Theflat panel display may in fact comprise one or more layers positionedsubstantially coplanar and adjacent to upper and/or lower surface ofsaid display substrate.

The term upper and lower surface of the first substrate should in thiscontext be construed a first and second surface, which in context withis to be construed as the larger surfaces of the first substrate or thedisplay substrate.

The display substrate according to the first aspect of the present mayfurther have a modulus of elasticity smaller than or equal to a modulusselected among the group consisting of 1.3 GPa, 1.1 GPa, 1 GPa, 0.9 GPa,0.8 GPa, 0.7 GPa, 0.6 GPa, 0.5 GPa, 0.4 GPa, 0.3 GPa, 0.2 GPa and 0.1GPa.

The flexible flat panel display according to the first aspect of thepresent invention may further comprise a first spacer and a secondspacer positioned between the first substrate and the display substrate,and a cell structure for containing the electro-optical medium anddefined between the first substrate, the display substrate, the firstspacer, and the second spacer. The cell structure may define a cell gapbetween the first substrate and the display substrate.

The flexible flat panel display according to the first aspect of thepresent invention may further comprise a first layer positionedsubstantially coplanar and adjacent to the first substrate, which firstlayer may have a modulus of elasticity, E_(I), and which may have athickness of up to 80% of the total thickness of the first substrate andthe first layer, and the first substrate may have a modulus ofelasticity, E_(II), where E_(I) is larger than E_(II).

In addition, the flexible flat panel display according to the firstaspect of the present invention may comprise a second layer positionedsubstantially coplanar and adjacent to the display substrate, whichsecond layer may have a modulus of elasticity, E_(III), and which mayhave a thickness of up to 80% of the total thickness of the displaysubstrate and the second layer, and the display substrate may have amodulus of elasticity, E_(IV), where E_(III) is larger than E_(IV). Byestablishing a composition of layers and substrates a particularlyadvantageous flat panel display is achieved since this increases bendingflexibility of flat panel displays manufactured according to knownprocedures. By for example coating a liquid crystal display with arubber foil on the outer surfaces of the first layer and/or second layerthe cell gap variation's sensitivity to bending may be significantlyreduced. Addition of the extra (=soft) layer significantly improves thebending (in)sensitivity and only slightly affects the bendingflexibility.

The ratio E_(I)/E_(II) and/or the ratio E_(III)/E_(IV) may be largerthan a number chosen among 2, 2.5, 3, 5, 8, 10, 15 or 20. The firstlayer and the second layer may be positioned nearest the electro-opticalmedium and the first substrate and the display substrate may bepositioned furthest from the electro-optical medium. Since tensilestrain in a bent flexible flat panel display is largest at the outermostsurfaces the design of the flexible flat panel display should ensurethat the modulus of elasticity is lowest at the outermost surfaces. Theflexibility of the flexible flat panel display and the stressesparticularly on the seal are reduced by the first substrate and thedisplay substrate having a lower modulus of elasticity.

The flexible flat panel display according to the first aspect of thepresent invention may be adapted to bend into a curvature, whileensuring a relative variation of the cell gap, Δ/d, equal to or smallerthan 5%. The flexible flat panel display satisfies the expression:${\Delta/d} \leq \frac{( {\frac{1}{d} + \frac{1}{h}} )L^{4}}{\kappa_{Geo}R^{2}h}$where d is the cell gap, h is thickness of the first or the secondsubstrates, L is the distance between the first and second spacers,κ_(Geo) is a geometric constant, and R is radius of the curvature of theflat panel display while bent.

The geometric constant, κ_(Geo), may equal a value in the range 1 to 64,such as for example κ_(Geo) equal to 32 when the flat panel display hassymmetrical first and second substrates. The value is dependent on thegeometrical shape of the flat panel display.

When d<<h then the expression may be reduced to: $\begin{matrix}{\Delta \leq \frac{L^{4}}{\kappa_{Geo}R^{2}h}} & \lbrack 2\rbrack\end{matrix}$where Δ is the actual cell gap variation.

It is a further object of the present invention to ensure a cellthickness (gap) control so as to reduce the cell gap deviation caused byin-homogeneities and/or bending to lesser 5% over each cell pitch. Theterm cell pitch is in this context to be construed as the distancebetween the spacers, or the span length between the spacers.

The relative cell gap variation according to the first aspect of thepresent invention may be equal to or smaller than a relative cell gapvariation selected among the group consisting of 5%, 4%, 3%, 2.5%, 2%,1.5%, 1%, 0.5%, 0.25% and 0.1%. The smaller the cell gap variation thebetter is the overall quality of the liquid crystal display and inparticular the overall quality of the information displayed on theliquid crystal display.

The first substrate and/or the display substrate according to the firstaspect of the present invention may comprise a flexible polymer beingtransparent or opaque. Polymer failure should preferably be avoided thusthe first substrate and/or display substrate with lower modulus providesfor a high failure strain, as opposed to substrates with higher modulus,giving a low failure strain.

The substrate according to the first aspect of the present invention maybe bendable into a radius of curvature smaller than a radius selectedamong the group consisting of 300, 200, 100, 50, 40, 30, 20, 15, 10, 5,3 and 1 mm. Selecting the first substrate and/or display substratehaving low modulus of elasticity provides for a flexible flat paneldisplay being bendable into a small radius of curvature. Therefore incase a larger radius of curvature is a design criteria then the choiceof materials for the first substrate and/or the display substrates maybe selected having higher modulus of elasticity. Selecting a materialwith low modulus of elasticity may provide for a flexible flat paneldisplay with reduced shear loading on the seal line of the flexible flatpanel display cells, and a lowered risk of failure of the cells.

The provision of a bendable substrate ensures that for example aflexible flat panel display maybe folded around a wide variety ofobjects thus increasing the usability of flat panel displays. In factthe bendable substrate may be used for carrying electronic circuitry forany handheld and mobile devices and thus provide a unique possibility ofimproving the utilisation of space in such devices.

The flexible flat panel display according to the first aspect of thepresent invention may further comprise a plurality of first and secondspacers between the first substrate and the display substrates defininga plurality of cell structures there between.

The above objects, together with numerous other objects, advantages andfeatures, which will become evident from below detailed description, isobtained according to a second aspect of the present invention by aflexible substrate characterised in having a modulus of elasticitysmaller than or equal to 1.5 GPa.

The flexible substrate according to the second aspect of the presentinvention may incorporate any features described with reference to thefirst aspect of the present invention.

The above objects, together with numerous other objects, advantages andfeatures, which will become evident from below detailed description, isobtained according to a third aspect of the present invention by amethod for manufacturing a flat panel display, which method may compriseadding a first and second layers to a regid flat panel display, thefirst and second layers having a low modulus of elasticity such asrubber foil. Alternatively, the method may comprise providing a firstand second layer in coplanar relationship, adding spacers between thefirst and second layer so as to define there between a cell structurefor receiving a electro-optical medium, adding a additional layerspositioned substantially coplanar and adjacent to the outermost surfacesof the first and second layers and having a modulus of elasticitysmaller than the first and second layers.

The method for manufacturing a flexible flat panel display according tothe third aspect of the present invention may incorporate any featuresdescribed with reference to the first and second aspect of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional features and advantages of the presentinvention, will be better understood through the following illustrativeand non-limiting detailed description of preferred embodiments of thepresent invention, with reference to the appended drawings, wherein:

FIG. 1, shows a schematic representation of a cross section of asubstrate;

FIG. 2, shows a graph of critical values for height as a function ofwall-separation (pitch) for a bi-layer cell with substrates with equalparameters; and

FIG. 3, shows a schematic representation of a cross section of a part ofa flexible LCD.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic representation, 10, showing a cross section of afirst substrate, 12, carrying two spacers, 14 and 15. The figure is notto scale. The parameters L, d and h are indicated in the figure. L isthe span length, i.e. the distance between the middle of the spacers. dis the initial cell gap, and h is the thickness of the substrate.

FIG. 2 depicts critical values for height as a function of pitch, for abi-layer cell with equal parameters, where Δ=0,1 μm. The solid lines arecalculated for bent cells fixed R, while the broken lines are calculatedfor flat cells at constant applied pressure q and fixed E. The workingarea for proper functioning upon both pressure application and bendingin this figure is the upper left corner.

FIG. 3 is a schematic representation of a cross section of a part of aflexible LCD, 110, according to the present invention. The figure is notto scale. The flexible LCD, 110, comprises a first substrate 122 and afirst layer 124, and a display substrate 128 and a second layer 126. Thefirst layer 124 and second layer 126 are separated by a number ofspacers, 142, 144 and 146. The first layer 124 has a modulus ofelasticity larger than the first substrate 122, and the second layer 126has a modulus of elasticity larger than the display substrate 128.

EXAMPLES Example 1

The spacer pitch in a matrix addressed flexible LCD display ispreferably chosen in accordance with the pixel pitch. The cell gapvariation upon bending depends on substrate height and spacer pitch(equation 1). In the present example an allowed cell gap change Δ=0.1 mmis assumed for the working display. For 150 mm thick substrates andspacer pitch L=500 μm a minimum bending radius R=11.4 mm is obtained. Atthis radius the maximum strain in the substrate is 1.3%.

The use of a high modulus substrate (E=2000 MPa) is compared with a lowmodulus substrate (E=100 MPa).

At a critical elongation the substrate behaves no longer elastically,either by onset to plastic deformation or due to mechanical failure. Thecritical elongation for high modulus materials is in general in theorder of 1%, while for low modulus materials it is above 10%.

The critical deformation limit (1%) of the LCD display with high modulussubstrates is achieved at R=15 mm. For the limit for the low modulussubstrate (10%) is achieved at R=1.5 mm.

At equal curvature the stresses in the display with low modulussubstrates are significantly lower (the ratio of the moduli of thesubstrates, in this example a factor 20). This reduces the risk offailure of for instance the peripheral seal line.

With low modulus substrates significantly lower bending radii can beachieved. The failure risk of displays or display elements, likesubstrates or seal, is strongly reduced.

Example 2

An LCD is made with high modulus (E=2000 MPa) thin (h=50 mP) substrates.The spacer distance in the cell L=500 μm. For proper cell gap control,Δ<0.1 μm is required. Then the limiting bending radius R=20 mm.

To achieve a better bending performance a second layer (150 □m, lowmodulus; E=100 MPa), is added to both display surfaces. With this addedlayer a significantly better bending performance is obtained. Propercell gap control (Δd<0.1 μm) is achieved down to a bending radius R=9mm.

In the present cell geometry, optical layers like a polarizer andretarder, may be integrated in the high modulus substrate.

The addition of (a) soft layer(s) to the substrate improves the cellperformance upon bending.

Example 3

Handling of these thin foils is a tedious job. Wrinkling may damage thethin substrates; the local stresses at the wrinkles are above theallowed strain.

As an example an electro-luminescent display (OLED) will be used. Thedisplay is made on a thin, high modulus, polymer substrate (E=3000 MPa,h=30 μm). The actual display structure has a limited thickness (<4 μm)on top of the substrate; it includes all functional layers, also(brittle) hermetic coatings. A low modulus layer is applied on top ofthe display structure (E=100 MPa, h=200 mm). The addition at least onelow modulus layer to the thin substrate retards the wrinkle formation.

A number of layers in the display, such as the brittle hermetic coating,fail above a critical strain. With a critical strain of 0.5%, the 30 μmthick substrate can be rolled to a radius of 3 mm. With the addition ofthe low modulus layer on the outer surface (of the rolled display) evena lower bending radius can be achieved.

1. A flexible flat panel display comprising a first substratecharacterized in that said first substrate has a modulus of elasticitysmaller than or equal to 1.5 GPa.
 2. A flexible flat panel displayaccording to claim 1, wherein said first substrate has a modulus ofelasticity smaller than or equal to a modulus selected among the groupconsisting of 1.3 GPa, 1.1 GPa, 1 GPa, 0.9 GPa, 0.8 GPa, 0.7 GPa, 0.6GPa, 0.5 GPa, 0.4 GPa, 0.3 GPa, 0.2 GPa and 0.1 GPa.
 3. A flexible flatpanel display according to claim 1, wherein said flat panel displaycomprises an electro-optical medium such as a liquid crystal, or anelectro-chrome or electro-phoretic element, a light emitting element, anorganic or inorganic light emitting element, polymer light emittingelement, or any combination thereof.
 4. A flexible flat panel displayaccording to claim 1 further comprising one or more layers positionedsubstantially coplanar and adjacent to upper and/or lower surface ofsaid first substrate.
 5. A flexible flat panel display according toclaim 1 further comprising a first layer positioned substantiallycoplanar and adjacent to said first substrate, which first layer has amodulus of elasticity, E_(I), and said first substrate has a modulus ofelasticity, E_(II), where is E_(I) larger than E_(II).
 6. A flexibleflat panel display according to claim 5, wherein said first layer ispositioned nearest said electro-optical medium and said first substratefurthest from said eletro-optical medium.
 7. A flexible flat paneldisplay according to claim 5, wherein the ratio E_(I)/E_(II) is largerthan a number chosen among the group of numbers 2, 2.5, 3, 5, 8, 10, 15or
 20. 8. A flexible flat panel display according to claim 5, whereinsaid first layer has a thickness of up to 80% of the total thickness ofsaid first substrate and said first layer.
 9. A flexible flat paneldisplay according to claim 1, wherein said first substrate is bendableinto a radius of curvature smaller than a radius selected among thegroup consisting of 300, 200, 100, 50, 40, 30, 20, 15, 10, 5, 3 and 1mm.
 10. A flexible flat panel display according to claim 1 furthercomprising a display substrate positioned coplanar with said firstsubstrate, which display substrate has a modulus of elasticity smallerthan or equal to 1.5 GPa.
 11. A flexible flat panel display according toclaim 10, wherein said display substrate has a modulus of elasticitysmaller than or equal to a modulus selected among the group consistingof 1.3 GPa, 1.1 GPa, 1 GPa, 0.9 GPa, 0.8 GPa, 0.7 GPa, 0.6 GPa, 0.5 GPa,0.4 GPa, 0.3 GPa, 0.2 GPa and 0.1 GPa.
 12. A flexible flat panel displayaccording to claim 10 further comprising one or more layers positionedsubstantially coplanar and adjacent to upper and/or lower surface ofsaid display substrate.
 13. A flexible flat panel display according toclaim 10 further comprising a second layer positioned substantiallycoplanar and adjacent to the display substrate, which second layer hasmodulus of elasticity, E_(III), and said display substrate has a modulusof elasticity, E_(IV), where said E_(III) is larger than E_(IV).
 14. Aflexible flat panel display according to claim 13, wherein the ratioE_(III)/E_(IV) is larger than a number chosen among the group of numbers2, 2.5, 3, 5,8, 10, 15 or
 20. 15. A flexible flat panel displayaccording to claim 13, wherein said second layer has a thickness of upto 80% of the total thickness of said display substrate and the secondlayer.
 16. A flexible flat panel display according to claim 10 furthercomprising a first spacer and a second spacer positioned between saidfirst substrate and said display substrate, and a cell structure forcontaining said electro-optical medium and defined between said firstsubstrate, said display substrate, said first spacer and said secondspacer, said cell structure defining a cell gap between said firstsubstrate and display substrate.
 17. A flexible flat panel displayaccording to claim 16, wherein said second layer is positioned nearestsaid electro-optical medium and said display substrate is positionedfurthest from said eletro-optical medium.
 18. A flexible flat paneldisplay according to claim 16, wherein said flexible flat panel displaybeing adapted to bend into a curvature, while ensuring a relativevariation of said cell gap, Δ/d, equal to or smaller than 5%.
 19. Aflexible flat panel display according to claim 16, wherein said flexibleflat panel display satisfies the expression:${\Delta/d} \leq \frac{( {\frac{1}{d} + \frac{1}{h}} )L^{4}}{\kappa_{Geo}R^{2}h}$where d is said cell gap, h is thickness of said first or said secondsubstrate, L is the distance between said first and second spacers,κ_(Geo) is a geometric constant, and R is radius of curvature of saidflat panel display while bent.
 20. A flexible flat panel displayaccording to claim 16, wherein said relative cell gap variation is equalto or smaller than a relative cell gap variation selected among thegroup consisting of 5%, 4%, 3%, 2.5%, 2%, 1.5%, 1%, 0.5%, 0.25% and0.1%.
 21. A flexible flat panel display according to claim 10 furthercomprising a plurality of first and second spacers positioned betweensaid first and second substrates defining a plurality of cell structuresthere between.
 22. A flexible flat panel display according to claim 1,wherein said first substrate comprises a flexible polymer beingtransparent or opaque.
 23. A flexible flat panel display according toclaim 10, wherein said display substrate comprise a flexible polymerbeing transparent or opaque.
 24. A flexible substrate characterised inhaving a modulus of elasticity smaller than or equal to 1.5 GPa.
 25. Aflexible substrate according to claim 24, wherein said substrateincorporates any features of the flexible flat panel display accordingto claim 1.